The present invention relates to a dental crown and to a rigid connector for the crown to secure to a removable prosthetic tooth telescopingly in place, the securement occurring through parallel walls of the crown and the connector engaging functionally. The invention also concerns the rigid connector itself and concerns a ceramic core useful for precisely manufacturing such a crown.
Missing teeth can be replaced in the mouth by either a permanent or a removable prosthesis. The present invention addresses the removable type.
Missing teeth are replaced with removable structures. A partial prosthesis is secured to the remaining natural teeth so as to distribute the force of the bite between them and the tegument. A partial prosthesis comprises three elements, namely artificial teeth, a base, and structural elements. The structural elements constitute cushioning elements and connectors. The connectors are either fasteners, resilient connectors, pivoting connectors, or rigid connectors. The present invention concerns rigid connectors.
A characteristic of rigid connectors is an absence of play between the patrix or the permanent connector installed in the jaw and the matrix on the removable crown placed on the patrix. The matrix and the patrix accordingly combine in every application the functions of support, positioning, and skew prevention. Support is provided by an interior backing, while positioning and skew prevention are provided by ensuring a parallel interface between the matrix and the patrix.
Secure fastening is ensured either by friction between the matrix and the patrix or by additional active or passive fasteners. If the hold depends on friction between the matrix and the patrix, it can be restored when necessary by adjusting their mutual fit. What are called actively activatable connectors are employed. In other cases, either the patrix or the matrix, or both must be replaced to restore a secure hold. In this case, the connectors are called passively activatable.
Rigid connectors are bridges, attachments, and telescopes or double crowns. Rigid connectors in the form of double crowns combine the functions of hold, positioning, skew prevention, and thrust distribution. Holding is ensured in parallel walled telescoping connections by friction between the primary and the secondary components. Since friction is lost from a parallel walled telescoping connection, additional active and passive fasteners are needed, especially in the upper jaw. Appropriate structures include friction pins, bars, spherical springs, and snaps. These additional fasteners are intended to prevent an unstressed prosthesis from coming loose by itself.
Telescoping connections with parallel walls include closed, open, and partial telescoping connections. Telescoping connections with conical walls are called conical crowns and those connections are always closed.
Partial telescoping connections, which may also be called groove-and-shoulder telescoping or groove-shoulder-and-pin telescoping, are employed, particularly to replace front teeth and premolars. The lingual portion of the primary crown or matrix has a groove and shoulder machined into it, and the lingual tooth form is provided by this outer telescoping part or crown. The advantage of a partial telescoping connection is that removing the prosthesis reveals the faced primary telescoping part, and appearance is not affected as it is in the case of full telescoping connections. One drawback of partial telescoping connections is their relatively restricted friction area. They are therefore usually provided with additional friction pins that rest resiliently against the primary crown to augment and prolong the securement. Experience has demonstrated, however, that the additional friction pins break off very rapidly due to material stress and fatigue, when anatomical considerations dictate that they be less than 3 mm long.
All three types of telescoping connections must be made individually by the dental technician. Prefabrication is currently impossible. Since the securement that counteracts the tension and compression accompanying the process of mastication depends only on the friction between the patrix and the matrix, all the cooperating surfaces on the patrix and matrix must be parallel and free of play. Since the connectors are small, the technician must use miniature tools, which undesirably yield resiliently to the resulting pressures.
These drawbacks to the individual manufacture of telescoping connections increase as the heights of the telescoping connections decrease. Partial telescoping connections with parallel walls will no longer function when they are less than approximately 4 mm high.